The present invention relates to a plasma processing apparatus and, more particularly, to a surface wave plasma processing apparatus which is suited for etching a sample, such as a semiconductor element substrate having a resist used as a mask, with a plasma and for ashing the same to remove the resist.
In the magnetic-fieldless microwave plasma processing apparatus which uses microwaves, but has no external magnetic field for producing a main plasma, as disclosed in Japanese Laid Open Patent No. 83337/1991, a waveguide tube having a square or circular section is provided vertically with respect to a dielectric window of quartz through which the microwaves are introduced. Generally when the microwaves are introduced into a magnetic-fieldless plasma, it is known that microwaves having an angular frequency .omega. can not be propagated in a plasma having a critical electron density n.sub.c, as described in "Purazuma Butsuri Nyomon" (Introduction to Plasma Physics) written by F. F. Chen, translated by Taijiro Uchida and published in 1977 by MARUZEN, where: EQU n.sub.c =m.omega..sup.2 /4.pi.e.sup.2,
in which m is the mass of an electron and e is the electron charge. Microwaves having a frequency or 2.45 GHz cannot be propagated in a plasma having a critical electron density of 7.times.10.sup.7 /cm.sup.3 or more. In this situation, the plasma is so produced that the plasma seems to adhere to the vicinity (about several mm) of the dielectric window. Therefore, the aforementioned conventional construction having a waveguide tube provided vertically with respect to the dielectric window is made such that microwaves are introduced with the wave fronts of their electric field being generally in parallel with the plasma adhering to the dielectric window.
The aforementioned plasma processing apparatus gives no consideration to the reflection of microwaves from the plasma. When microwaves are introduced such that the wave fronts of their electric field are generally in parallel with the plasma adhering to the dielectric window and having a critical electron density of n, or more, they are reflected by the plasma. In the waveguide tube, therefore, standing waves having an eigenmode, such as a circular mode TE.sub.11 are generated according to the size and shape of the waveguide tube, as described in "Maikuro-ha Kougaku" (Microwave Engineering) written by Masamitsu Nakajima and published in 1975 by MORIKITA SHUPPAN. Since the distribution of the plasma to be produced agrees with the eigenmode of the microwaves, there arises a problem in that the plasma becomes nonuniform in the plane adhering to the dielectric window. Because a plurality of the aforementioned eigenmodes of the microwaves can be present, transition between the eigenmodes is caused by a small disturbance such as the output fluctuation of the microwaves. This causes another problem that the plasma becomes unstable.